The mosaic structure of molecular sieve crystals: understanding the growth process of molecular sieve crystals

Molecular sieves are one of the materials with far-reaching influence in materials science and catalysis in the past 50 years. Although the description of molecular sieves has a history of about 250 years, the past 50 years have been a period of vigorous development of molecular sieves. By 2013, more than two hundred molecular sieves with different structures have been successfully synthesized. The application of molecular sieves in different fields such as catalysis and cleaning affects each of us.



Although the structural characterization of molecular sieves can reach a very precise level, there is still a lot of room for discussion on its growth process and structural design at the nanoscale. Recently, the Valentin Valtchev research team of the Laboratory of Catalysis and Spectroscopy Chemistry (LCS) in France has reversed the growth mechanism of molecular sieves through fluorine-treated molecular sieves. This work was published on Angew. Chem. Int. Ed., the first author is Zhengxing Dr. Qin.



The author's previous work has proved that the use of HF-NH4F can quickly wash away the Si and Al of the molecular sieve, and the defect site is dissolved first, and thus the mesoporous structure can be formed. In this work, the author pointed out that the use of NH4F is not only safer, but also easier to control (NH4F can undergo double hydrolysis in solution, Figure 1).

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Figure 1. The balance of NH4F and the corresponding species in the solution. Image source: Angew. Chem. Int. Ed.



In the process of dissolving commercial and synthetic MFI molecular sieves, the author found that some of the more fragile parts were dissolved first, such as the particle interface or the boundary of continuous crystals (Figure 2, a~c). A more interesting phenomenon occurs in the subsequent dissolution. Unlike the previous dissolution from the periphery, the molecular sieve dissolves the mesopores from the inside to obtain a sponge molecular sieve (Figure 2, d). It is worth noting that under long-term dissolution, the remaining part of the molecular sieve still maintains a good crystallinity (see the original XRD and BET results).

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Figure 2. TEM images of molecular sieves after dissolution for different times. Image source: Angew. Chem. Int. Ed.



After close observation, the author found that these dissolving positions all left rectangular pores of 10-20 nm, and these pores all have similar orientation and size (Figure 2, e). With the increase of time, the pore size gradually When it becomes larger, the small holes are connected to become large holes (Figure 2, f~h). Electron tomography technology confirmed that these pores are square, and the pore volume and surface area can be quantified at different dissolution times (Figure 3).

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Figure 3. The electron tomography image of the sample dissolved in the corresponding direction for 15 minutes. Image source: Angew. Chem. Int. Ed.



This phenomenon appears in many other molecular sieves, but the story has not reached the core yet: What is the principle behind this phenomenon? The author pointed out that from the phenomenon that the direction and size of the dissolved pores are uniform, it can be seen that this dissolution actually originates from the removal of small crystal grains with definite boundaries from the large molecular sieve particles, which in turn deduces the growth mechanism of molecular sieve : 1) the formation of small nano-sized particles, 2) the accumulation and aggregation of particles, and 3) the formation of large-sized crystal grains (Figure 4). This is an important clue to explore the growth mechanism of molecular sieves.

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Figure 4. Molecular sieve growth (A) and dissolution process (B). Image source: Angew. Chem. Int. Ed.



In addition, the authors found that the mesopores produced by dissolution also contribute positively to the diffusion of substrates in catalytic reactions.



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