Mosaic structure of zeolite molecular sieve crystal

Zeolite molecular sieve is one of the most influential materials in materials and catalysis in the last 50 years.While on the description of the zeolite molecular sieve has a history of about 250 years, but nearly 50 years is zeolite molecular sieve in the booming period, 2013 - more than two hundred kinds of different structure of zeolite molecular sieve has been successfully synthesized.The application of zeolite molecular sieve in various fields such as catalysis and cleaning affects every one of us.
Although the structural characterization of zeolite molecular sieve has reached a very precise level, there is still a lot of space to explore the growth process and the structure design of nanometer scale.Recently, the French ENSICAEN catalysis and spectral chemistry lab (LCS) Valentin Valtchev team by working backward fluorine handling zeolite molecular sieves the growth mechanism of zeolite molecular sieve, the work is published in the Angew. Chem. Int. Ed., the first author is Zhengxing Dr Qin.
Previous work has shown that the use of HF-NH4F can quickly wash the skeleton Si and Al of zeolite molecular sieve, dissolving the defective bits first, and therefore forming the mesoporous structure.In this work, the author points out that the NH4F is not only safe, but also easier to control (NH4F can double hydrolysis in solution, figure 1).

Figure 1. Equilibrium and corresponding species of NH4F in solution.Photo source: Angew. Chem. Int. Ed.

Dissolved in business and in the process of synthesis of MFI zeolite molecular sieve, the author found that some of the more vulnerable parts of the first to dissolve, such as particle interface or even had the crystal boundary (figure 2, a ~ c).
More interesting phenomenon occurred in the next dissolve, unlike before dissolved from the periphery, zeolite molecular sieve dissolved out from within mesoporous, get the sponge type zeolite molecular sieves (figure 2, d).
It is worth noting that, in the long dissolving, the zeolite molecular sieve of the residue remains very good crystallinity (see the XRD and BET results of the original text).

Figure 2. TEM pattern of molecular sieve after dissolving different time.Photo source: Angew. Chem. Int. Ed.
After close observation, the authors found that these dissolved after the left 10-20 nm rectangle holes, these holes and have similar size and orientation (figure 2, e), as the growth of the time, hole size increases gradually, and holes are linked together into a big hole (figure 2, f ~ h).
The electron tomography technology confirms that the holes are square and can be quantified at different time of dissolution (FIG. 3).

Figure 3. Electron tomography of samples that dissolve 15 minutes in the corresponding direction.
Photo source: Angew. Chem. Int. Ed.
This phenomenon is present in other zeolite molecular sieves, but the story still does not reach the core: what is the principle behind this phenomenon?
The author points out that from the dissolved pore, size is very uniform in the direction of the phenomenon can be seen that the dissolved in fact derived from boundary of zeolite molecular sieve particles with a small grain of removed, this is the launch of the growth of zeolite molecular sieve mechanism: 1) the nano-sized particles generated, 2) accumulation of particles and aggregation, 3) the large size of the production of grain (figure 4).
This is an important clue to explore the growth mechanism of zeolite molecular sieve.
In addition, the authors find that this dissolution produces a mesoporous contribution to the diffusion of substrate in the catalytic reaction.