Atomic-Scale Insights into Topotactic Transformations in an Extra-Large-Pore Zeolite Using Time-Resolved 3D Electron Diffraction

Nanoscience Chemistry Materials Science Inorganic Non-Metallic Materials Catalysis Chemical Engineering
zeolite topotactic transformation time-resolved electron diffraction extra-large-pore structure atomic scale materials engineering solid-state reactions

Academic Background

Zeolites are microporous materials with regular channel structures, widely used in catalysis, adsorption, and ion exchange. Due to their unique pore structures and chemical properties, zeolites hold significant application value in petrochemicals, environmental protection, and energy storage. However, the synthesis and structural control of zeolites still face numerous challenges, particularly in the synthesis and structural stability of extra-large-pore zeolites. Traditional synthesis methods often struggle to precisely control the structure of zeolites, while topotactic transformation, as an important strategy, can achieve the directional synthesis and structural control of zeolites through atomic-scale structural changes.

This study aims to reveal the topotactic transformation process of the extra-large-pore silicate zeolite ECNU-45 to ECNU-46 using time-resolved three-dimensional electron diffraction (3D ED) technology. By analyzing the structural dynamics at the atomic scale, the researchers hope to gain a deeper understanding of the structural change mechanisms during the topotactic transformation of zeolites, thereby providing new insights for the engineering design and synthesis of zeolite materials.

Source of the Paper

This paper was co-authored by Yi Luo, Hao Xu, Yue Han, and others, affiliated with Stockholm University, East China Normal University, and Sinopec Shanghai Research Institute of Petrochemical Technology. The paper was published in Nature Synthesis in April 2025, with the DOI 10.1038/s44160-024-00715-1.

Research Process and Results

1. Synthesis and Structure Determination of Zeolites ECNU-45 and ECNU-46

The researchers first synthesized two extra-large-pore silicate zeolites, ECNU-45 and ECNU-46. ECNU-45 features a three-dimensional interconnected 24×10×10-ring channel system, while ECNU-46 consists of a one-dimensional 24-ring channel system connected to 10-ring channels. The synthesis of ECNU-45 employed 1,1,6,6-tetramethyl-1,6-diazacyclododecane-1,6-diium hydroxide (TDDH) as the organic structure-directing agent (OSDA) and was crystallized from a high-concentration synthesis gel. ECNU-46 was obtained through the topotactic transformation of ECNU-45 under acidic conditions.

Using three-dimensional electron diffraction technology, the researchers determined the crystal structures of ECNU-45 and ECNU-46. Both zeolites have similar unit cell parameters and space group (P-62c), but their framework structures exhibit significant differences. The three-dimensional channel system of ECNU-45 is formed by the interconnection of 24-ring and 10-ring channels, while the one-dimensional 24-ring channel system of ECNU-46 is connected via 10-ring channels. Structural analysis revealed that the framework structures of both ECNU-45 and ECNU-46 are constructed from columnar building units arranged along the c-axis, but they differ in specific atomic positions and connectivity.

2. Time-Resolved 3D Electron Diffraction Reveals the Topotactic Transformation Process

To uncover the topotactic transformation process from ECNU-45 to ECNU-46, the researchers employed time-resolved three-dimensional electron diffraction technology to conduct detailed structural analysis of the reaction intermediates. ECNU-45 was treated under acidic conditions (HCl/EtOH/H2O, 1 M), and 3D ED data of the reaction intermediates were collected at different time points (0, 1, 2, 4, 6, 8, 10, and 24 hours).

Through the 3D ED data, the researchers found that the topotactic transformation process involves six independent tetrahedral silicon sites (T7-T12), accompanied by atomic displacement, addition, and removal, as well as bond formation and breakage. Specifically, during the reaction, silicon atoms at the T7 site gradually shifted to the T10 site, and new silicon atoms were introduced at the T9 and T11 sites. Subsequently, silicon atoms at the T11 site moved to the T12 site, and finally, silicon atoms at the T8 and T9 sites were removed. These structural changes led to the gradual transformation of the three-dimensional channel system of ECNU-45 into the one-dimensional channel system of ECNU-46.

3. Analysis of the Topotactic Transformation Mechanism

By combining scanning electron microscopy (SEM), powder X-ray diffraction (PXRD), solid-state 29Si magic-angle spinning nuclear magnetic resonance (MAS NMR), and thermogravimetric analysis (TGA), the researchers further analyzed the mechanism of the topotactic transformation. The study revealed that the topotactic transformation process can be divided into three stages: In the first stage, approximately half of the OSDA was rapidly removed from the channels, and some silicon atoms at the T7 site shifted to the T10 site, while new silicon atoms were introduced at the T9 and T11 sites. In the second stage, the remaining OSDA was almost entirely removed, more silicon atoms at the T7 site shifted to the T10 site, and silicon atoms at the T11 site moved to the T12 site. In the third stage, the remaining silicon atoms at the T7 site continued to shift to the T10 site and reacted with silicon atoms at the T12 site until the T10 and T12 sites were fully occupied.

4. Stability Analysis of Zeolite Structures

The researchers also calculated the stabilization energies of ECNU-45, ECNU-46, and RZM-3 (a zeolite with the same framework structure as EMM-23) using density functional theory (DFT). The results indicated that ECNU-45 has lower stability, primarily due to the low occupancy rates at the T8 and T9 sites. In contrast, the framework structure of ECNU-46 remained stable under calcination at 550°C, while the framework structure of ECNU-45 collapsed after the removal of the OSDA.

Research Conclusions and Significance

This study, for the first time, revealed the topotactic transformation process of the extra-large-pore zeolite ECNU-45 to ECNU-46 at the atomic scale using time-resolved three-dimensional electron diffraction technology. The results demonstrate that the topotactic transformation process involves atomic displacement, addition, and removal, as well as bond formation and breakage, ultimately leading to the reconstruction of the zeolite channel system. This research not only provides new insights into the synthesis and structural control of zeolites but also offers important atomic-scale information for understanding complex solid-state reaction mechanisms.

Research Highlights

  1. Atomic-Scale Structural Dynamics Analysis: This study, for the first time, revealed the structural changes during the topotactic transformation of zeolites at the atomic scale, providing a new perspective for understanding solid-state reaction mechanisms.
  2. Time-Resolved 3D Electron Diffraction Technology: The researchers developed and applied time-resolved 3D ED technology, successfully capturing the structural changes of reaction intermediates, offering a new tool for studying the structural dynamics of complex materials.
  3. Stability Control of Zeolite Structures: Through topotactic transformation, the researchers successfully synthesized ECNU-46 zeolite with a stable framework structure, providing new possibilities for the engineering applications of zeolite materials.

Other Valuable Information

The experimental data and crystal structure information from this study have been made publicly available. The researchers also provided detailed supplementary materials, including experimental methods, data analysis, and computational details. These data and information serve as important references for subsequent research.

Through the in-depth analysis of this study, the synthesis and structural control of zeolite materials will usher in new development opportunities, particularly in the application prospects in petrochemicals and environmental protection.