Recent advances in understanding the mechanisms of zeolite crystallization: a review

Crystallization mechanism and crystal growth pathways of zeolites

By studying the crystallization mechanism of zeolites, the essence of the crystallization process of zeolites can be understood, which provides theoretical guidance for controlling their physical properties and synthesizing new zeolite materials. Since Barrer et al. first proposed the hydrothermal synthesis mechanism of zeolites in 1959^[35], researchers have never stopped exploring this aspect^[36]. It should be noted that due to the diversity of zeolite materials and synthesis conditions, their crystallization mechanisms also vary. Even for the same zeolite, its crystallization mechanism may vary depending on changes in the synthesis system and conditions. The crystallization mechanism can be divided into solution-mediated transport mechanism and solid hydrogel transformation mechanism based on whether the liquid phase components directly participate in crystallization.

Solid hydrogel transformation mechanism

In 1968, Flanigen and Breck put forward the solid hydrogel transformation mechanism^[43]: the initial silicon aluminum gel is generated after the mixing process of raw materials, and the zeolite crystal is gradually formed by depolymerization and rearrangement under the condition of hydrothermal crystallization. In the crystallization process, the solid phase of gel does not dissolve, and the species in the liquid phase do not directly participate in the nucleation and growth of zeolites. In 1989, Xu et al. successfully synthesized ZSM-5 and ZSM-35 zeolites in non-aqueous solutions^[44]. The results showed that the Si/Al ratio remained unchanged in the solid phase after crystallization, and silicate or aluminate ions did not exist in the liquid phase, strongly supporting the solid hydrogel transformation mechanism. Furthermore, in the process of synthesizing zeolites using the solvent-free method, no additional liquid water was added, except for a small amount introduced with the raw materials and reagents. This was also a typical example of the solid hydrogel transformation mechanism^[45].

Shao et al. pretreated the synthesis system of TS-1 and prepared hierarchical porous TS-1 using an extremely viscous solid gel. The authors studied changes in the synthesis system and the process of zeolite nucleation and growth. They concluded that the synthesis of hierarchical pore TS-1 follows the solid hydrogel transformation mechanism [Figure 1]^[46]. Similarly, Li et al. synthesized hierarchical porous ZSM-5 using silica spheres as raw material. Research has shown that silica spheres are not only nutrients, but also guide the formation of macroscopic/microscopic pore structures, consistent with solid hydrogel transformation mechanism^[47].

Figure 1. Illustration of the proposed formation mechanism of the single-crystalline hierarchical zeolites^[46]. Copyright © 2021 Elsevier Inc. All rights reserved.

Researchers have found that, during the crystallization process of zeolites, some phenomena cannot be well explained by a single solid hydrogel transformation or solution-mediated transport mechanism.

Equilibration mechanism

In 1981, Derouane et al. proposed the biphasic transformation mechanism^[48]. This viewpoint suggested that both solid-phase and solution-mediated transformation coexisted during the crystallization process of zeolites, and they could occur separately in two crystallization reaction systems or simultaneously in one system. Iton et al. discovered through small-angle neutron scattering that different silicon sources led to various conversion mechanisms during the synthesis of ZSM-5^[49] and proposed that the crystallization process was a combination of multiple reaction mechanisms. Under varying crystallization conditions, even the same zeolite would follow different mechanisms.

Besides the issue of which phase crystallization occurs, the growth of crystals has become the next concern for researchers. There are two theories describing the mechanism of zeolite growth: (1) the classical theory of crystallization based on spontaneous nucleation and growth from the addition of silicate molecules; and (2) the non-classical theory of crystal nucleation and growth involving the direct aggregation and attachment of nanoparticles. The schematic diagram of two crystallization pathways is shown in Figure 2^[50].

Figure 2. Schematic representation of classical and non-classical crystallization. (A) Classical crystallization pathway; (B) oriented attachment of primary nanoparticles forming an iso-oriented crystal upon fusing; (C) mesocrystal formation via self-assembly of primary nanoparticles covered with organics^[50]. Copyright © 2005 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

Classical routes usually involve spontaneous nucleation and are derived by the addition of monomers (atoms or molecules) from the growth solution to kink, step, and terrace sites on crystal surfaces, resulting in crystals with smooth surfaces^[51]. Non-classical processes encompass the addition and attachment of precursors, which range from oligomers to primary particles or even fully developed nanoparticles, comprising disorder-to-order transformation processes, leading to the formation of crystals with an irregular morphology, rough crystal surfaces, and mesoscopic structures^[52,53]. To determine which mechanism the zeolite crystallization process follows or to enable theoretical innovation, a detailed characterization of the synthesis system and its products is required. With the progress of analytical science, multiple advanced characterization methods are now available.

Characterization of zeolite crystallization process

Ex-situ characterization

In most cases, the ex-situ method is used to characterize the crystallization mechanism, which has the characteristics of convenience and low instrument requirements, bringing good operability. The combination of multiple methods can comprehensively characterize the products or intermediate at each stage of crystallization and thus infer the complete crystallization process.

X-ray diffraction (XRD) is the most important tool for studying zeolites. The crystal planes of zeolites act as gratings to diffract X-rays, allowing the distinction of the crystal planes through diffraction peaks. These characteristic peaks are then used to determine the topological structure of the zeolite. When characterization conditions are consistent, the XRD spectrum can be used to calculate the relative crystallinity of zeolites, providing a guarantee for studying crystallization kinetics. From the relative crystallinity obtained via XRD, it is found that almost all hydrothermal synthesized zeolites follow the Avrami equation during their crystallization process. This process consists of induction periods, crystal growth periods, and completion periods^[54]. Based on this, the zeolite crystallization process can be understood as an auto-catalytic reaction, where, upon heating, the raw material generates an active center for crystallization, and subsequent crystallization occurs around this active center.

Apart from XRD, scanning electron microscopy (SEM) and transmission electron microscopy (TEM) are important fundamental characterization methods for materials. The material states of various crystallization stages can be visually characterized with SEM, including raw materials, amorphous precursors, and crystallized zeolites. TEM provides local structural information of small species. High-resolution TEM (HRTEM) typically offers a resolution below 1 Å, enabling the direct observation of material defects, structures, and other local structural information at the atomic scale, as well as simultaneous electron diffraction analysis. TEM can also be used as evidence to distinguish between classical and non-classical pathways. Meanwhile, field emission SEM with energy dispersive X-ray (FE-SEM-EDX) spectroscopy is an effective method for visually characterizing the surface element distribution (content and uniformity) of zeolites through electron microscopy^[55]. In summary, electron microscopy can track the entire process from the initial raw material to the complete zeolite, intuitively describe the nucleation and growth of zeolite, and provide element distribution information for relevant stages. It plays an important role in characterizing the crystallization mechanism of zeolites.

Fourier transform infrared spectroscopy (FT-IR) and ultraviolet (UV)-Raman techniques can be applied to the study of zeolite synthesis and provide structural information of intermediate species during the crystallization process. In particular, UV-Raman can effectively identify the SBU structures in zeolite and zeolite synthesis systems. It is worth noting that UV Raman spectroscopy is also frequently used in the study of zeolite transformation to elucidate the decomposition or retention of certain structures^[56].

Solid-state nuclear magnetic resonance (SS NMR) is an effective technique for studying local structural changes and the chemical environments of atoms. As an analysis method that does not destroy samples, nuclear magnetic resonance (NMR) is an indispensable means to analyze the structure of zeolites. Ex-situ NMR can maximize the use of commercial instruments, thus having the characteristics of convenient testing and reproducible results. Meanwhile, multiple pulse sequences [e.g., one-pulse, cross-polarization (cp), rotational-echo double-resonance (REDOR)] and 2D spectra [heteronuclear correlation (HETCOR), multiple-quantum (MQ) magic angle spinning (MAS), double quantum-single quantum (DQ-SQ), heteronuclear multiple-quantum correlation (HMQC)] can reveal the chemical environment of various atoms in zeolite and reveal their interactions^[57].

Dynamic nuclear polarization (DNP) is a branch of NMR spectroscopy that utilizes the electron nucleus double resonance principle to excite free electron transitions in microwaves, polarizing the spin energy level distribution of related nuclei. This can significantly enhance the sensitivity of NMR methods and provide information on microscopic electronic structures. Berkson et al. investigated the crystallization process of siliceous MFI zeolite and investigated the meta-structured MFI zeolite nanosheets using DNP-enhanced 2D J mediated ²⁹Si{²⁹Si} NMR experimental [Figure 3] proving that the mesostructured MFI zeolite nanoplates formed from non-topological layered sites, which clearly established the coefficient con activities of transforming intermediate nanolayered site motifs and crystallizing mesostructured MFI zeolite nanosheets^[58]. Although the experiments have shown that DNP technology significantly improves detection sensitivity and obtains structural information, the limited number of instruments and high experimental difficulty currently make the research in this area insufficient.

Figure 3. Solid-state 2D DNP-enhanced J-mediated (through-bond) ²⁹Si{²⁹Si} correlation spectrum of the intermediate product of crystallizing MFI zeolite^[58]. Copyright © 2017 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim. DNP: Dynamic nuclear polarization.

Although the exome representation methods have the aforementioned advantages, they cannot avoid the possible changes that may occur during the material annealing or quenching stage, which may lead to the obtained results not reflecting the true state of reaction crystallization. Meanwhile, certain characterization and sample preparation processes inevitably influence the physicochemical properties of intermediates in the crystallization, even after the completion of zeolite crystallization. As analytical science advances, various in-situ characterizations have been applied to the study of zeolite crystallization processes.

In-situ characterization

To address the above-mentioned challenges and make the relevant characterization more accurate in reflecting the system and intermediate products during the crystallization process, various in-situ methods have been used. This includes in-situ XRD, in-situ NMR, in-situ infrared (IR) spectroscopy, and in-situ Raman spectroscopy, among others. Among these, in-situ NMR is considered the most important and promising tool for researching the crystallization mechanism of zeolites.

Due to the particularity of NMR instruments and the effectiveness of NMR characterization, some researchers were committed to in-situ NMR characterization of reaction systems. Due to the unique nature of hydrothermal systems, instruments were required to meet the requirements of the “three resistances” - high-temperature resistance, high-pressure resistance, and alkali corrosion resistance. For this purpose, they designed and manufactured some instruments or equipment accessories, and successfully characterized the synthesis system by in-situ NMR. Since the first in-situ characterization of zeolite A in 1996^[59], relevant work has been mainly on materials with low silicon-aluminum ratios and low-temperature crystallization. In addition, some atomic nuclei have lower natural abundance and weaker signals, requiring longer sampling times, which contradicts the original intention of in-situ characterization - time resolution. Balancing time resolution and spectral resolution is also a major challenge in in-situ characterization. Characterizing high-temperature, high-pressure, and highly corrosive systems remains a challenge in relevant research.

With the advancement of materials science and the development of nuclear magnetic instruments, in-situ instruments used in zeolite synthesis processes are gradually developing; some special devices for in-situ NMR have been manufactured and successfully characterized for the zeolite crystallization process^[59-63] [Figure 4]. The basic trends of the development are as follows: (1) From specialized probes to commercial probes, the requirements for instruments are reduced; (2) From liquid probes to solid probes, the magic angle rotation improves the spectral resolution; (3) Increasing the synthesis temperature from low to high expands the range of materials that can be characterized; (4) Just as an increase in temperature leads to a rise in pressure, transitioning from low to high pressure also expands the range of materials that can be characterized; (5) From natural to enriched isotopes, some nuclei, such as ²⁹Si and ¹³C, have lower natural abundance and can only provide extremely low signals within a limited sampling time. Isotope labeling can greatly improve their signal-to-noise ratio and save sampling time^[64]. Although in-situ characterization of hydrothermal synthesis processes has been achieved, more scenes of in-situ NMR application are dry gel conversion (DGC). This is because the pressure of DGC is lower, reducing the risk of instrument damage. The application of in-situ SS NMR technology in the study of zeolite crystallization mechanism still requires significant effort.

Figure 4. Sample tube for in-situ NMR characterization of crystallization process. (A) The first MAS NMR cell sused for in situ studies of hydrothermal synthesis of zeolite sreported by Shi et al. Copyright © 1996, American Chemical Society^[59]; (B) Quartz glass ampoules by Liu et al. Copyright © 2009 Elsevier Inc. All rights reserved^[60]; (C) HPHT cell for liquid state NMR. HPHT MAS NMR cells suitable for in situ NMR studies of zeolite synthesis under hydrothermal conditions. Copyright © 2000 Wiley^[61]; (D) PEEK insert for Bruker 7 mm MAS rotor. Copyright © 2006 Elsevier Inc^[62]; and (E) 7.5 mm ZrO₂ rotor for Chemagnetic/Varian MAS probe. Copyright © 2016, American Chemical Society^[63]. NMR: Nuclear magnetic resonance; MAS: magic angle spinning; HPHT: high pressure high temperature; PEEK: polyether ether ketone.

In addition to in-situ NMR, in-situ IR and in-situ Raman spectroscopy have also been used to characterize the crystallization of zeolites. In situ IR can observe the changes in functional groups in synthetic systems and is used to describe the process of changes in the bonding relationships of atoms. In situ Raman can observe the Raman vibration in the system, which is mainly used to describe the formation of SBUs and frameworks. Due to the equipment limitations, synthetic systems with a crystallization temperature around 100 °C are often chosen for in-situ IR and in-situ Raman characterization. Xue et al. monitored the crystallization of LTA zeolite from an organic silicon source using in-situ IR spectroscopy. The in-situ IR analysis revealed the appearance of Si-OH, and based on this, the authors proposed a crystallization mechanism^[65]. Fan et al. monitored the crystallization of Zeolite X^[66] and AlPO-5^[67] using in-situ Raman, described the changes in Al-P bonding relationships and the structure of organic templates, and proposed the crystallization mechanism of AlPO-5. In addition to hydrothermal systems, this method is also applicable to the dry gel method^[68] and the synthesis of metal-organic frameworks (MOFs) in solvothermal systems^[69].

As technology advances, some other characterization methods have been extended into the field of zeolite synthesis. For example, conductivity testing [alternating current (AC) impedance spectroscopy] is commonly used for characterizing battery electrolytes^[70] and photo density waves are used for studying material particle size^[71]. These methods provide another perspective for studying the zeolite crystallization process. All the above experiments demonstrate that applying some other characterization methods to zeolite synthesis systems can reflect certain laws in zeolite synthesis and can serve as a supplement to XRS, SEM, TEM, and NMR.

Metal cations

Silcoaluminate zeolites are generally synthesized in alkaline systems, where high alkalinity is beneficial for dissolving silicon species in raw materials or accelerating the decomposition of organic materials. In this way, when adding inorganic bases, it is inevitable to introduce metal cations. Those commonly used include alkali metal ions Li⁺, K⁺, Na⁺, and alkaline earth metal ions Ca²⁺ and Ba^2+[75]. Overall, metal cations play a role in regulating the pH of the synthesis system, guiding the structure, and balancing the charge of the framework.

As is well known, the difference between zeolites and amorphous aluminosilicates lies in the fact that zeolites have a regular framework and pore structure, and metal ions can balance the charge of framework acid sites. Wang et al. investigated the crystallization of ZSM-57. It was observed with TEM that the use of different inorganic cations could lead to the formation of different twins. By regulating the growth route, the size of twin single-crystal chips was adjusted, revealing the reasons for the formation of morphological differences. Experiments have shown that the use of different inorganic cations leads to different crystallization mechanisms. In the K⁺ system, the classical solution-mediated pathway dominates the whole crystallization process while the classical solution-mediated path in the initial stage and the subsequent non-classical solid-mediated path are identified in the Na⁺ system^[76].

Metal cations commonly used as inorganic template agents, such as Na⁺, have the characteristics of high natural abundance and strong nuclear magnetic response. Researchers can easily track changes in the chemical environment during the crystallization process, providing a powerful tool for studying the crystallization mechanism of zeolites. Yamada et al. studied the differences between the precursors of FAU zeolite and glass with a similar composition. Regarding the crystallization behavior of FAU, NMR analyses revealed that the ²³Na environment changed slightly due to a gradient in order during the crystallization and an increased Na–O bond distance [Figure 5]. Furthermore, based on pair distribution function (PDF) patterns, the fraction of small-membered rings (e.g., 4Rs) was found to be higher in the zeolite precursor. The authors believed that Löwenstein’s rule, which prohibited the formation of Al–O–Al bonding, was also substantially applicable to the structure of the zeolite precursor^[77].

Figure 5. ²³Na MQ-MAS-NMR of (A) glass and zeolites with different crystallization times, (B) 3 h, (C) 10 h and (D) 24 h^[77]. Copyright © 2018 Elsevier Inc. All rights reserved. MQ: Multiple-quantum; MAS: magic angle spinning; NMR: nuclear magnetic resonance.

In addition to the type of cation, the amount of cation also influences the zeolite crystallization process. Chawla et al. used the FE-SEM-EDX method to investigate MER, MFI, MOR, TON, and LTL zeolites. The intermediate states during the crystallization process were characterized. The authors monitored the initiation of nucleation, completion of crystallization, and presence of impurities using FE-SEM-EDX. Ex-situ experiments have demonstrated that sufficient differences in chemical composition (such as alkali metal content) between amorphous precursors and crystals can be used to (i) track the early stages of zeolite crystallization, (ii) locate its occurrence, and (iii) distinguish residual amorphous materials in the crystallization domain^[78].

The above experiments indicate that different metal ions play different roles in complex zeolite synthesis systems and interlayer conversion processes. Their chemical environment becomes irregular as synthesis progresses, and during the zeolite conversion process, some framework aluminum atoms are considered important active centers and driving forces.

Inorganic anions

In hydrothermal synthesis systems, inorganic cations serve multiple roles. However, previous research on the role of anions is not sufficient and mainly focuses on hydroxide and fluoride ions. In recent years, research on anions has begun to expand to more inorganic anions.

Sun et al. studied the effect of inorganic anions in the S-1 zeolite synthesis system. The sodium salts of the Hofmeister series promoted the crystallization of silicalite-1, in the order of Na₂SO₄ > NaF > NaCl > NaBr > NaI > NaSCN. The research proved with liquid ¹H-NMR that the ions were located around the template with a distance to the N atom of TPA⁺. The authors believed that the anions affected the release of water molecules by activating the hydration spheres around the TPA⁺ cation to a different degree, thus regulating zeolitic crystallization^[79].

Moreover, they introduced halogen anions into the synthesis system of MOR and studied their influence on the crystallization process. Theoretical calculations [Figure 6] showed that iodine ions could accelerate the formation of Si–O–Si bonds and hinder the formation of Si–O–Al. Experimental results showed that this strategy indeed improved the silicon-aluminum ratio of MOR, verifying the theoretical calculation results. The authors conducted other experiments to prove that this rule was also applicable to the crystallization of NaY and ZSM-5, and the obtained zeolite had an improved silicon-aluminum ratio^[80]. This is an application of the mechanism described above. In parallel, Wang et al. conducted a similar work in the synthesis system of ZSM-12 zeolite and obtained identical conclusions. The obtained zeolite, as a precious metal carrier, exhibited excellent performance in the hydrogenation isomerization reaction of n-dodecane^[81].

Figure 6. Gibbs free energy surfaces of (1)-(4) at 453 K and 1 atm obtained by DFT theoretical calculation^[80]. Copyright © The Royal Society of Chemistry 2022. DFT: Density functional theory.

The above experiment proves that inorganic anions influence the zeolite synthesis system through their interaction with organic cation templates. This interaction increases the rate of Si–O–Si bond formation while reducing the rate of Si–O–Al bond formation, thereby enhancing the Si/Al ratio of the obtained zeolite. This effect is primarily due to the charge density and degree of charge non-uniformity in the zeolite synthesis system.

Organic amines and quaternary ammoniums

Starting from the early 1960s, organic compounds mainly composed of amines have been used as templates for zeolite synthesis. Organic molecules or quaternary ammonium and quaternary phosphine cations have more possibilities in terms of size, shape, and functional groups compared with spherical inorganic cations^[82]. Organic chains have complexity, flexibility, and selectivity, making the synthesis system of zeolites complex. So far, except for some zeolites synthesized with metal cations as structural directing agents, most zeolites have been synthesized with organic compounds as template agents.

The use of organic templates can adjust the pH of the synthesis system and synthesize the zeolites that cannot be synthesized with metal ions as templates. When the topological structure is the same, the use of organic template agents can improve the silicon-aluminum ratio of the synthesized zeolite. Because of the charge of amines and ammonium, they interact with the framework and can balance the framework charge and affect the product framework charge density. In addition, to obtain a single phase, some other molecules can act as growth inhibitors to inhibit the formation process of certain phases in mixed-phase synthesis systems. However, the use of organic compounds can cause wastewater and exhaust gas problems, making template-free synthesis an important research direction.

The general understanding of the role of organic templates in structure orientation during the hydrothermal process is that, under the heating of gel/dry gel, the raw material species and/or their solvates surround the template^[83], followed by the nucleation and growth of the zeolites. Due to differences in the structure and polarity of organic matter, gel concentration, the initial silicon, aluminum and/or phosphorus species, the hydrolysis rate, and the crystallization temperature, the interaction between organic matter and raw materials varies. This variation leads to differences in the structure and properties of the synthesized zeolites, providing a theoretical basis for regulating zeolite properties by controlling the synthesis process.

Nishitoba et al. investigated the process of synthesizing CHA-type silicoaluminate zeolites using FAU and LTL zeolites as raw materials and trimethyladamantyl ammonium hydroxide and tetraethyl ammonium hydroxide as organic templates. The authors conducted detailed monitoring of crystallization behavior, and the experiment proved that organic structure-directing agents (OSDA) greatly affected the composition of the final products, and that the raw materials strongly influenced the Al distribution in the final products. The results obtained from this study clearly showed that the raw materials for the CHA-type zeolite would be optimized depending on its application. The methanol to olefins (MTO) performance of the obtained zeolite improved, compared to traditional CHA zeolite^[84].

In-situ SS NMR is an effective characterization method for the zeolite crystallization process. Ivanova et al. monitored the crystallization of Beta under in-situ conditions using NMR and enhanced the signal of silicon species and template agents using ²⁹Si and ¹³C isotopes. The experiment proved that the solution-mediated transport and solid hydrogel transformation mechanisms were followed in the two synthetic systems, respectively. Similarly, NMR also demonstrated the different structural orientation behaviors of the template in the two reaction pathways. As shown in Figure 7A, using in-situ¹³C MAS NMR, the authors proved that there were methyl TEA⁺ cations in the pores of BEA zeolite, TEA⁺ interacting with silicon species gel and TEA⁺ clusters in the reaction gel system. This indicated that in the crystallization process, the template performed multiple functions simultaneously, such as combining with the raw materials, balancing the zeolite framework charge and stabilizing the gel. The synergy of these effects demonstrated the complexity of the synthesis system and crystallization process. The results indicate that MAS NMR can serve as a superior tool for monitoring the hydrothermal synthesis of various solids (including zeolites, zeolite types, mesoporous materials, MOFs, etc.), and can also be used to design new excellent materials for different applications^[85]. Similarly, Zhao et al. used in-situ NMR technology to characterize the changes in ³¹P,²⁷Al, and ¹H spectra during the crystallization process of AlPO-5 zeolite synthesized at 150 °C. The results showed that phosphorus and aluminum species existed in the form of Al-O-P single chains after initial condensation and continued to condense under the action of a template to form a semi-crystalline chain structure with alternating 4R and 6R, ultimately forming a framework structure with 12-membered ring pores^[63].

Figure 7. (A) Representation of the mechanism of zeolite BEA crystallization^[85]. Copyright © 2017 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim; (B) Schematic representation of the proposed mechanism for formation of the AlPO-11 framework^[86]. Copyright © 2023 The Authors. Published by American Chemical Society.

In-situ NMR can characterize the gel system and conduct in-situ hydrothermal synthesis; however, due to the above reasons, especially the limitations of high-temperature resistance, high-pressure resistance and alkali corrosion resistance of the instrument, the current in-situ NMR characterization still focuses on the crystallization of DGC. Alahakoon et al. studied the crystallization mechanism of the DGC AlPO-11 using a commercially available 5 mm high pressure high temperature (HPHT) rotor. NMR tracked the chemical environment changes of Al and P atoms, and characterized the evolution of the framework structure. ¹H → ¹³C cross-polarization (CP) tracked the role of organic structure directing agents, and ¹H was used to reveal the effect of water content on crystallization kinetics [Figure 7B]. Experiments showed that the interaction between water and structure-directing agents (SDA) played an important role in the zeolite crystallization process, which was difficult to achieve through ex-situ characterization. The authors believed that combining the ex-situ and in-situ results would provide a clearer, more comprehensive, and less ambiguous picture of the formation of molecular sites^[86].

In addition to in-situ NMR, Raman spectroscopy and simulated annealing can also be used to study the host-guest interactions during zeolite synthesis. Yuan et al. studied the crystallization mechanism of SAPO-34 synthesized with the hydrothermal method using TEA as a template. They first detected 4R and 8-membered rings (8R, 8MR) through Raman spectroscopy, while 6R appeared slightly later, indicating that the formation of cha cages occurred first. d6r units developed during the cage combination connection process^[87]. Yan et al. synthesized four CHA silicoaluminophosphate zeolites (SAPO-34/44) using different OSDAs and studied the host-guest interactions using XRD and simulated annealing^[88].

The above experiments prove that the interaction between raw materials and organic templates in gel is of positive significance, especially for the adjustment of aluminum distribution in zeolite. However, recent research has shown that organic template agents not only interact with raw materials and/or their solvates, but also have interactions with metal ions. In some systems, even interactions between template agents exist. These effects result in significant differences in the physicochemical properties of the synthesized zeolite.

Park et al. investigated the synthesis process of Al-CON and B-CON zeolites under hydrothermal conditions. The experiment indicated that the CON nucleus contained Al(SiO)₄ specifications and the crystal growth was processed by incorporating B atoms into the CON structure together with the reorganization of the tetrahedral boron silicate specifications. The authors believed that although the details on the reactions between liquid and the solid phases during the crystallization could not be fully understood, they could discriminate the different crystallization stages in the solid phase^[89]. Meanwhile, the authors found that during the crystallization of CON, the amount of Na⁺ in the solid gradually decreased over time, while the quantity of organic template agents gradually increased, which was reflected in the results of thermogravimetric analysis and inductively coupled plasma atomic emission spectroscopy (ICP-AES) [Table 1]. This showed the different interactions between metal ions and organic compounds at varying stages of crystallization. Similarly, Sada et al. synthesized FAU zeolite at 180 °C using TPA⁺ cations and crystal seeds and studied the crystallization mechanism. Experiments showed that seed crystals and Na⁺ played a critical role in the FAU secondary nucleus after the solution of seeds, and TPA⁺ stabilized the FAU framework through the formation of an organic-organic complex during crystal growth^[90].

Some organic compounds can alter the crystal growth pathway of zeolites. Kumar et al. used scanning probe microscopy to in-situ monitor the crystallization of zeolte LTA with a spatiotemporal resolution that captured dynamic processes in real time. The experiment showed a new crystal growth pathway, which involved the formation of gel-like islands from supersaturated solutions compared to (aluminum) silica molds. The author believed that LTA crystallization occurred through multiple pathways^[91]. Later, the authors replaced Na⁺ with TMA⁺, leading to the non-classical growth pathways of LTA zeolite. They provided unprecedented insights into the surface growth of zeolite with molecular addition using a time-resolved microscope, which directly captured the occlusion of silica nanoparticles and emphasized the universal role of defects in zeolite crystallization^[92]. These results demonstrated the synergistic effect of metal cations and organic compounds in the synthesis of certain zeolites, providing new insights into the role of template agents.

Recent studies have shown that the interactions between template agents themselves can also affect the crystallization process of zeolites, mainly through hydrogen bonding. Liu et al. investigated the synthesis process of SSZ-13 and ITQ-1 zeolites using 2D ¹H DQ-SQ. Experiments demonstrated specific spatial correlations between the TMAda⁺ cations and the SiO-H-OSi hydrogen bonds in the frameworks of SSZ-13 and ITQ-1, indicating that the electronic interactions between the TMAda⁺ cations and the negative charge centers in the framework oriented the crystallization process. The authors also studied SAPO-11 and SAPO-34. Two-dimensional ¹H DQ-SQ confirmed that two organic amines were H-bonded in each unit cell. It could be suggested that the hydrogen bonds between organic amines in the channels were the driving force for the formation of zeolites^[93]. This result indicated that in studying the crystallization process of zeolites, the interaction between the template and the framework atoms and the interaction between the template itself should be considered. Afterward, they used the same method to study S-1 zeolite, and the experiment proved that the SiO…H-OSi hydrogen bonds between lamellae were formed and gradually transformed into the in-cage ones during the crystallization process. The SiO…H-OSi hydrogen bond between lamellae acted as a connector to assemble the silicate species together to generate the zeolite framework^[94].

In conclusion, organic templates exist in various forms in gel, such as existing independently, interacting with raw materials, forming cluster molecules in gel, interacting through hydrogen bonds, and existing in formed zeolite channels. Experimental results have shown that there are various host-guest interactions related to template agents in zeolite synthesis systems. These interactions between the host and guest lead to the complexity of the synthesis system, affecting the structural orientation of the template agent. The crystallization process of zeolite is a comprehensive effect of these factors, and in-depth research on the host-guest interactions remains an important aspect of future research on the zeolite crystallization mechanism.

Other organic additives

In addition to organic amines and quaternary ammonium template agents with structural directing effects, many organic molecules, while unable to act as structure-directing agents, can modify the crystal growth process of zeolites. These molecules, known as crystal growth regulators, can adjust the pore properties of zeolites and influence the atomic distribution within their frameworks. Specifically, some form complexes with transition metals, facilitating their incorporation into the zeolite framework. Others form micelles in the gel to become hierarchical pore-forming agents. Certain surfactants alter the aggregation state of the gel, thus changing the crystallization mechanism of the zeolite. In summary, adding organic compounds to zeolite systems is an effective means of regulating zeolite performance.

Some substances decompose, producing compounds that affect the synthesis system and alter the crystal growth pathway of zeolites. Qin et al. added arginine to the synthesis system of S-1 zeolite, switching the dominant mode of growth from non-classical to classical pathways. The experiment proved that arginine decomposed into guanine in the synthesis system, inhibiting non-classical pathways^[95].

In the above experiment, arginine switches the crystal growth pathways of S-1 from non-classical pathways to classical pathways. Similarly, other substances can switch the crystal growth path from classical to non-classical pathways. Zhang et al. added 1,3,5-benzenetricarboxylic acid (H₃BTC) to the synthesis system of TS-1 to regulate the distribution of titanium species. H₃BTC converted the precursor gel to the solid-liquid mixed state, leading to the transformation of the traditional liquid phase mechanism to solid-liquid combined mechanism to form TS-1 crystals. The performance observed in the oxidation of 1-hexene was objectively improved^[96]. After that, they changed the crystal growth pathways of TS-1 zeolite from classical to non-classical pathways by adding polyacrylamide (PAM) to the TS-1 zeolite synthesis system. They also demonstrated the diffraction pattern of zeolite precursors under the non-classical pathways through TEM. At the same time, The PAM-regulated TS-1 zeolite exhibited enhanced catalytic performance in oxidative reactions^[55]. Afterward, they replaced polymer PAM with acrylamide (AM) monomers and other similar substances, regulating the distribution of framework titanium species while adjusting the zeolite particle size. The authors attributed this to the interaction between monomers and titanium species and the self-polymerization of AM. The obtained zeolites exhibited excellent performance in ODS reaction^[97]. Overall, these studies underscore the complexity of elucidating the role of modifiers in zeolite synthesis. Experiments have shown that organic additives can alter both the crystallization mechanism and the crystal growth pathways.

Some cationic polymers, such as polydimethyldiallyl ammonium chloride (PDDA) and polyhexamethylene biguanide hydrochloride (PHMB), can also regulate the growth of zeolites. Unlike quaternary ammonium salts, their organic parts are so large that they cannot function as structural directing agents. In the synthesis system, they mainly act as surfactants, affecting the crystallization process of zeolites by adjusting the colloid chemistry of the synthesis system. In 2022, Dai et al. discovered that PDDA can accelerate the crystallization of SSZ-13 and adjust the particle size by several orders of magnitude. Light attention techniques demonstrated that polymers in the synthetic system accelerated the aggregation of amorphous substances, accelerated nucleation, and thus altered the particle size of zeolites. A lower polymer concentration can achieve high-speed synthesis of SSZ-13 at a very low cost^[98]. In 2024, Ren et al. synthesized TS-1 with a short B-axis by adding PDDA to the synthesis system. Experiments had shown that PDDA significantly increased the titanium content on the surface of zeolites, boosted the amount of hexacoordinated titanium, and reduced the formation of anatase. The obtained zeolites exhibit excellent performance in ODS^[99]. Research had shown that PHMB exhibits strong coordination with aluminum species in the synthetic system, resulting in the presence of hexacoordinated Al³⁺ species in the micropores of SAPO-11 zeolite and regulating the crystal orientation of the zeolite to form plate-like SAPO-11^[100]. A similar effect was observed in MFI (ZSM-5, S-1, and TS-1), where PHMB allowed more Al and Ti to enter the framework, adjusting the crystal orientation to form plate-like MFI zeolite^[101]. Even in the template-free synthesis of the ZSM-22 system, PHMB, as the only organic compound, created a hierarchical pore structure and adjusted the aluminum distribution of the zeolite, reducing the concentration of Brønsted acid^[102].

Unlike the additives mentioned above, some additives can accelerate zeolite nucleation by interacting with template agents. Yang et al. added N-methyl-2-pyrrolidone (NMP) to the synthesis system to shorten the crystallization time of ZSM-11. It was found that NMP could facilitate the bonding of TBA⁺ with aluminosilicate by breaking the hydration layer and accelerating the nucleation process. The authors studied the effects of NMP content, silicon-aluminum ratio, and synthesis temperature on crystallization and proposed an acceleration mechanism of NMP. To verify the accuracy of this mechanism, the authors repeated the experiment using N-ethyl-2-pyrrolidone (NEP) as a replacement for NMP. The observed ZSM-11 exhibited similar hydraulic stability to conventional ZSM-11^[103].

In a word, organic additives play multiple roles in the synthesis system of zeolite. The way they influence the crystallization mechanism and crystal growth pathway depends on their form in the synthesis system and their role in the composition of the gel. However, whether acting independently or synergistically, these additives are widely used to regulate the synthesis process of zeolites in order to control the properties of the synthesized materials. Therefore, in-depth research on the crystallization mechanism and zeolite crystallization pathway under the presence of organic additives, especially understanding their form and mechanism of action, is of great significance for the directional design of zeolites with high application performance.

Free radicals

After discussing the impact of organic additives, the next topic of discussion is free radicals, discovered in the past decade, that have specific effects on silicon species during the crystallization process. These radicals are a type of substance with a single electron and active chemical properties and can be introduced into the synthesis system of zeolite by using UV irradiation, free radical initiators, and Fenton reagents. The introduced free radicals can be easily characterized using electron paramagnetic resonance (EPR) spectroscopy. Experimental results have shown that in zeolite synthesis systems, free radicals can accelerate the breaking of old Si–O–Si bonds and the formation of new ones, significantly shortening the induction period of zeolite crystallization^[104]. This principle has been applied in aluminosilicate zeolites, pure silica zeolites, ordered mesoporous materials, and silicon aluminum phosphate zeolites.

Specifically, in 2016, Feng et al. found that hydroxyl radicals, generated through UV irradiation and Fenton reagents, could accelerate the breaking of old silicon oxide silicon bonds and the generation of new ones. Guided by this strategy, they accelerated the synthesis of a series of silicon aluminum zeolites, including NaA, NaX, NaZ-21, and silicalite-1^[104,105]. Later, Wang et al. added hydrogen peroxide as a free radical initiator to the synthesis system of zeolite Y, successfully increasing the silicon-aluminum ratio of zeolite Y without the use of organic templates, and as shown in Figure 8, the signal of free radicals can be detected by EPR^[106].

Figure 8. EPR spectrum of initial synthesis gel containing spin-trapping agent DMPO in the synthesis system^[104]. Copyright © 2016, American Association for the Advancement of Science. EPR: Electron paramagnetic resonance; DMPO: 5,5-dimethyl-1-pyrroline N-oxide.

Some researchers have introduced this conclusion into the synthesis system of silicon aluminum phosphate zeolites. Zhang et al. added potassium persulfate to the synthesis system of SAPO-18 zeolite, greatly accelerating its crystallization. Experimental results showed that extremely low concentrations of potassium persulfate (nK₂S₂O₈:nAl₂O₃ ≈ 10^-3) could accelerate the crystallization of SAPO-18, and the introduction of free radicals could enhance the acidity of the zeolite. The MTO performance of the obtained samples was improved^[107]. Zhou et al. recognized that silicon species acted as the crystallization centers of SAPO-34 zeolite when free radical initiators were added to the synthesis system. Experimental results showed that free radical initiators had the optimal concentration and could effectively shorten the crystallization induction period of SAPO-34 zeolite^[108], which confirmed previous conclusions^[109].

In summary, whether in the silica-alumina zeolite or the phosphate silica-alumina zeolite system, free radical initiators accelerate the activity of silicon species in the system and increase the crystallization rate of the zeolite. Under the assistance of free radicals, high crystallinity zeolites can be produced within the same duration. There are various ways to introduce free radicals into zeolite systems, and the operation is simple and convenient. This method has great potential for expansion and application.

Host-guest interaction in IZC process

When the raw material used for synthesizing zeolite is zeolite with a different topological structure, this method is called the IZC method. This involves the fracturing of the topological structure of the old zeolite and the formation of new topological structures in the zeolites. Typically, the IZC process can be divided into the following four stages: (I) incongruent source dissolution; (II) an equilibrium or induction phase with steady compositions; (III) crystallization toward a crystalline material; and finally, (IV) maturation of fully crystalline materials. During this process, the smallest building units formed by the decomposition of old zeolites and their role in the formation of new zeolites have always been a focus. Additionally, research has focused on the host-guest interactions in the IZC process, such as how old zeolites decompose under the influence of new template agents, and how the “structural fragments” formed after decomposition recombine to form new zeolites.

Undoubtedly, in the direct crystallization process, “non framework” metal ions act as templates and charge balancers. However, in the IZC process, Al species serve as the driving force in IZC synthesis, with possible effects including charge, dissolution, concentration, and mobility. Devos et al. investigated the IZC of FAU to MFI. For comparison, they also studied equivalent crystallization systems without Na⁺ and compared the differences between the IZC process and the SSZ-13 zeolite crystallization process. The authors believed that the key to this study was temporal synthesis profiles while probing the distribution and evolution of proximal acid sites with divalent cation capacity measurements. The author detected changes in local charges, distinguishing between Al-loving and Al-averse systems, enabling a new degree of control over the acidity and ion-exchange properties of zeolites^[110]. This provides an important reference for us to understand the role of metal ions in the conversion process of zeolite.

In addition to metal cations, water is also an important factor in the IZC process. Zhang et al. investigated the IZC process from FAU to CHA and MFI. They monitored the transformation process of SBUs using UV Raman spectroscopy. Experiments have shown that D6R is an important intermediate. For the IZC from FAU to CHA, D6R is directly converted, while for the conversion from FAU to MFI, it is first decomposed into single 6Rs (S6Rs), which is then assembled into the MFI framework. Meanwhile, 5Rs are only observed in the MFI framework formation, suggesting that the SBUs in the transformation of FAU to MFI are S6Rs compared to 5Rs^[111]. Later, they also studied the transformation process of FAU, MFI, and ^*BEA into AEI structures. The authors investigated the process of water decomposing zeolite framework and forming corresponding hydroxyl forms through theoretical calculations and calculated their hydrolysis energies. The results indicate that when FAU is used as the starting zeolite, the D6R units will be directly converted to AEI zeolite, as both FAU and AEI zeolites share the same D6R units. Meanwhile, during the conversion process of MFI and ^*BEA zeolites, the S5R units in the framework are decomposed into S4R and S6R units, which can be reassembled into D6R units and then crystallized into the AEI zeolite in the system. The authors believe that these insights into molecular-level interlayer transformation provide a foundation for developing new routes for efficient zeolite synthesis^[112].

In the above IZC process, zeolite decomposes in the synthesis system and is retained as SBU or larger units. However, recent research suggests that the IZC process does not necessarily follow this exact pattern. Li et al. synthesized ZEO-2, a kind of complex chain silica zeolite precursor using ricyclohexylmethylphosphonium (tCyMP) as OSDA. There were super large 16-membered rings (16-MR) and 14-membered rings (14-MR) in ZEO-2, and there were a large number of silicon hydroxyl groups in the chain structure, which were connected to each other by hydrogen bonds. The authors achieved the transformation from 1D to 3D structure by heating ZEO-2. During the calcination process, the single 4R (S-4R) silanol groups connected to each other in ZEO-2 condensed into D-4R in ZEO-3. In this process, the main structure of ZEO-2 was preserved. This discovery has challenged concepts deeply rooted in zeolite science, and opened up the possibility of chain silicates as precursors for the crystallization of zeolites^[15]. After the condensation to construct the zeolite was completed, the author changed the strategy and linked the layered structure of ZEO-2 through organic silicon dimethyldichlorosilane (DCDMS) or 2,4,6,8-tetramethylcyclotetrasiloxane (TMCTS), forming ZEO-4A and ZEO-4B. After removing the organic matter through calcination, ZEO-5 with a triple 4R (T4R) structure was obtained [Figure 9]^[16]. Although the ring tension is high, the authors demonstrated the stability of T4R through NMR.

Figure 9. Preparation and structure of ZEO-4A, ZEO-4B and ZEO-5^[16]. Copyright © 2024, The Author(s), under exclusive licence to Springer Nature Limited.

These two examples utilize the host-guest interaction between guest molecules and zeolite frameworks to synthesize new structures and raise new questions about the conversion process between zeolites. Specifically, it is necessary to consider whether the zeolite structure of the raw material needs to be completely separated during the zeolite conversion process. Additionally, what are the largest structural units that can be retained during zeolite conversion and how are they preserved? Addressing these questions will be the focus of future research on the IZC process. Based on Chen’s strategy, the development of more extra-large pore zeolite structures is anticipated in the future.

Dynamics research based on macroscopic phenomena

Generally speaking, the crystallization curve of zeolite shows an “S” shape. During the early stages of crystallization, there is a considerable period during which crystals cannot be detected by XRD, known as the induction period. After crystallization begins, the crystallization rate gradually increases, which is a self-catalytic process. As the process continues, the rate gradually slows down, and then the crystallization curve tends to flatten out. With ongoing research, the Avrami equation has been proposed to describe the crystallization process of crystals.

Chen et al. synthesized ZSM-5 zeolite using the leached illite clay by organic template-free and solvent-free methods. The authors investigated its crystallization behavior and kinetics at different temperatures. According to calculations, the apparent activation energies for nucleation and growth were 61.0 and 70.9 kJ/mol, respectively^[113]. Similarly, the crystallization process of NaA zeolite according to solid-state theory also follows the Avrami equation, and in-situ Raman spectroscopy revealed the transformation process of SBUs during the crystallization process. Experiments showed that mesoscale intermediates were formed in the aging stage of gel, primarily with a double quaternary ring and beta cage structure. The synthesized zeolite had excellent exchange performance^[114].

Although some researchers have also fitted the Avrami equation based on relative crystallinity, their understanding of the crystallization process varies. Corregidor et al. investigated the crystallization kinetics of ZSM-5 synthesized under hydrothermal conditions at different temperatures using natural perlite as a silicon aluminum source. The relative crystallinity of ZSM-5 zeolite was obtained by using XRD and the crystallization curve was plotted. The results indicated that the crystallization curve conformed to the nonlinear Avrami equation, and the authors calculated kinetic constants (k), Introduction periods (t0), and Avrami’s components (n). Based on this, A five-step crystallization mechanism has been proposed [Figure 10]^[54]. Wang et al. investigated the MOR crystallization. Unlike traditional segmentation, they divided the crystallization process into three stages: acute, rapid growth, and slow growth stages. This was because they found that during the stage where the relative crystallinity no longer increased, the aluminum atoms of MOR underwent directional migration from the 8MR to the 12-membered rings (12MR), leading to stronger acidity and decrease in catalytic performance for dimethyl ether carbonylation reaction^[115]. The above experiments show that although many researchers have different understandings of the crystallization process, their crystallization kinetics curves conform to the Avrami equation.

Figure 10. Crystallization curves of ZSM-5 zeolite at different temperatures and the schematic diagram of crystallization mechanism^[54]. Reprinted (adapted) with permission from Corregidor P, Acosta D, Destéfanis H. Kinetic study of seed-assisted crystallization of ZSM‑5 zeolite in an OSDA-free system using a natural aluminosilicate as starting source. Ind Eng Chem Res 2018;57:13713-13720. Copyright © 2018 American Chemical Society.

Apart from XRD, the conductivity of zeolite synthesis systems can also be used to characterize the crystallization kinetics of zeolites. Pellens et al. designed a device for in-situ measurement of conductivity and characterized the precursor single-phase hydrogenated silica ionic liquids used for the synthesis of gismondine (GIS) zeolites. The testing of conductivity could characterize the crystallization kinetics of zeolites. The authors proposed a crystallization model to explain the results based on a surface growth mechanism, which derived the experiment indicates zeolite crystallization from highly ionic media proceeds via a multi-step mechanism, involving an initial reversible surface condensation of a growth unit, followed by incorporation of that unit into the growing crystal. A new crystallization model based on surface growth mechanism was proposed to explain the experimental results. The experimental results show that the crystallization of zeolite in high ion media is carried out by a multi-step mechanism^[70].

In addition to conductivity, in-situ kinetic characterization of zeolite synthesis systems also includes photon density waves (PDW). Häne et al. used PDWs to in-situ monitor the crystallization process of zeolite A. The onset and end of the crystallization process could be detected online and in real time. The reduced scattering coefficient, being a measure for particle number, size, and morphology, provided distinct process information, including the formation of ammonium particles and their transfer into crystalline zeolite structures. During the crystallization of zeolite A, the system exhibited maximum light scattering at a specific moment, indicating the formation of zeolite A crystals. Following this, the reduced scattering coefficient increased rapidly and eventually stabilized a constant value. The authors suggested that real-time determination of zeolite synthesis endpoint using PDW spectroscopy could reduce synthesis time, improve spatiotemporal yield, and eliminate the need for time-consuming in-process sampling. Finally, they investigated the applicability of PDW spectra under more stringent chemical and physical conditions by monitoring the synthesis of zeolite L^[71].

In the above discussion, researchers have studied the macroscopic dynamics of the crystallization mechanism. Beyond the traditional nonlinear Avrami equation, new insights have emerged. In addition to conductivity and PDW, advanced characterization methods from other fields are being introduced into the study of zeolite crystallization mechanisms, further deepening researchers’ understanding. Recently, attention has shifted toward exploring the relationship between macro dynamics and microstructure, with researchers seeking to uncover the connection between these aspects and identify the true driving force behind the crystallization process.

Microscopic driving force of crystallization

The zeolite synthesis system is complex, and the driving force may vary between different systems. However, there is a consensus among researchers that the driving force is related to the microstructure of the zeolite synthesis system. This may involve uneven element distribution within the zeolite precursor or the formation of SBU structures during the early stage of crystallization. Li et al. investigated the nucleation/crystallization process of nanoscale ZSM-5 crystals with aluminum partitioning using HRTEM and EDX. They discovered that amorphous particles with aluminum-rich edges were formed in the initial stage of zeolite growth, with sizes between 40 and 80 nm. Afterward, the nanoparticle crystallized into structures with an amorphous core and crystal shell. Experiments showed that nucleation occurred preferentially at the edges of particles. Subsequently, the rapid propagation of the lattice towards the core resulted in particles with a fully crystalline structure. The high aluminum content at the edges of the particles seemed to be the driving force for the nucleation and growth of ZSM-5 zeolite^[116].

The uneven distribution of aluminum above explains well the driving force behind the crystallization of MFI zeolite under non-classical pathways. However, other researchers propose that the crystallization driving force of ZSM-5 comes from the uneven distribution of hydroxyl groups. Zhang et al. extracted protozeolite (PZ) from the growth solution before the crystallization of MFI. These amorphous particles, approximately 8-10 nm in size, were rich in Si-OH, exhibited short-range ordering, and possessed microporous properties. The researchers then subjected PZ to ball milling and stamping treatment to produce integrated PZ (IPZ). Subsequently, under hydrothermal conditions, they synthesized ZSM-5 zeolite using PZ and IPZ as the sole raw materials and TPA as the template agent, thereby avoiding classical crystallization pathways. The authors characterized PZ, IPZ, and the resulting ZSM-5 using SEM, TEM, and SS NMR techniques. The results revealed that the crystallization process of ZSM-5 synthesized from PZ and IPZ followed non-classical pathways, with the microparticles composing ZSM-5 exhibiting diffraction patterns [Figure 11]. Ball-milling caused a change in the chemical environment of silicon hydroxyl groups. The authors believe that the oriented coalescence of the IPZ is achieved through condensation of high-density silanols, leading to anisotropic rates of non-classical growth. This finding is not only meaningful for studying non-classical crystallization pathways, but also establishes a new method for synthesizing zeolite nanosheets via silanol-engineered non-classical pathways^[117].

Figure 11. Proposed evolution process of MFI-type zeolite nanosheets^[117]. Reprinted (adapted) with permission from Zhang Q, Li J, Wang X, et al. Silanol-Engineered Nonclassical Growth of Zeolite Nanosheets from Oriented Attachment of Amorphous Protozeolite Nanoparticles. J Am Chem Soc 2023;145:21231-21241. Copyright © 2023 American Chemical Society.

For a long time, the formation process of SAPOs has been described as the incorporation of silicon atoms into the aluminum phosphate framework through isomorphic substitution through SM2 and SM3 mechanisms. However, this explanation fails to account for certain phenomena: for some topological structures, AlPO is easy to synthesize while SAPO is challenging to synthesize (such as AFN). Conversely, for other topological structures, SAPO is readily synthesized, but AlPO proves difficult (CHA). Therefore, even for the same topology, the crystallization mechanisms of AlPO/SAPO are likely different. Zhou et al. identified silicon species as the crystallization centers of SAPO-34 zeolite by adding free radical initiators to the synthesis system. Experimental results have shown that free radical initiators have the optimal concentration and can effectively shorten the crystallization induction period of SAPO-34 zeolite^[108], which confirms previous conclusions^[109]. Later, they verified these findings by adding polymers to the synthesis system, changing the gel concentration, and using the two-step crystallization process to characterize the early synthesis system using liquid NMR^[118]. Finally, they applied these insights to synthesize SAPO-34/SAPO-14 composite zeolites through a seed-assisted method^[119]. The resulting zeolites exhibited MTO performance. However, Zhou et al. have yet to address a problem: what are the composition and structure of the crystallization center of SAPO-34?

A few years later, this issue was resolved. Zhang et al. designed a top-down synthesis system for SAPO-34 zeolite and conducted extensive investigations. Using Fourier transform ion cyclotron resonance mass spectroscopy (FT-ICR-MS), they characterize the liquid precursors. Their experiments demonstrated that the crystallization centers of SAPO-34 are 4Rs and 6Rs with silicon aluminum structures^[120]. After that, they successfully synthesized SAPO-34 zeolite in the SAPO-34 synthesis system using a seed-assisted method with SAPO-18, SAPO-35, and SAPO-56 as seeds. The results indicate that D6R units are key features for a highly effective SAPO seed, potentially providing small fragments containing intact or broken d6r units with Si-O-Al domains after seed dissolution. These fragments then serve as the nucleus for the crystallization of SAPO-34. The obtained zeolite exhibits excellent catalytic performance in the MTO reaction. The authors believe this conclusion provides a theoretical basis for the synthesis of SAPO-34 molecular sieves by the seed-assisted method^[121].

Research in both early and recent literature primarily focuses on AlPO-5 and SAPO-34. AlPO-5 is considered the simplest aluminum phosphate zeolite, while SAPO-34 is the most important catalyst for the MTO reaction. The MTO catalytic performance of SAPO-34 is significantly influenced by its structure, and the crystallization mechanism is the factor determining the structure of SAPO-34 zeolite.

Factors affecting crystallization kinetics

Based on numerous examples, the factors that currently influence the crystallization mechanism and crystal growth pathways of zeolites can be summarized as follows: the concentration of the synthesis gel system, silicon content, raw material specifications, crystallization temperature, and even the order in which raw materials are added. The following summarizes the effects of these factors on the zeolite crystallization process.

Silicon content

Similar to gel concentration, the silicon content in some systems also affects the crystallization mechanism of zeolite. The increase in silicon content is essentially a result of increased gel concentration in the system. This has led to changes in the nucleation and growth. Jonscher et al. investigated the crystallization of ZSM-5, focusing on the correlation between the Si/Al ratio and zeolite morphology. They characterized zeolites with different Si/Al ratios and crystallization times. The results showed that a slow dissolution recrystallization process has been changed into a fast solid-state transformation with increasing Si/Al ratio. After that, small agglomerates began to assemble into bigger spherical particles [Figure 12A]. As a result, the performance of these two zeolites differs in methanol conversion experiment, which verifies that different crystallization mechanisms can lead to various zeolite structures, thereby exhibiting distinct catalytic performance^[123].

Figure 12. (A) Scheme of the mechanistic reason for the interdependency between Si/Al ratio and zeolite morphology of ZSM-5^[123]. Copyright © 2021 The Authors. ChemCatChem published by Wiley-VCH GmbH; (B) Schematic illustration of the crystallization pathways of TS-1 zeolite through classical and non-classical processes^[125]. Copyright © The Royal Society of Chemistry 2022; (C) Elemental mapping images of EU-45^[127]. Copyright © 2022 Elsevier Inc. All rights reserved.