What Is the Smallest Zeolite That Could Be Synthesized?**

Abstract

Zeolites with a few unit cells are promising as catalyst and adsorbents. The quest to synthesize the smallest zeolites has recently resulted in 4 to 8 nm nanozeolites, about 2 to 4 unit cells. These findings pose the question of what is the smallest zeolite that could be obtained by hydrothermal synthesis. Here we address this question using molecular simulations and thermodynamic analysis. The simulations predict that amorphous precursors as small as 4 nm can crystallize zeolites, in agreement with the experiments. We find that interfacial forces dominate the structure of smaller particles, resulting in size-dependent compact isomers that have ring and pore distributions different from open framework zeolites. The instability of zeolites smaller than 3 � 0.5 nm precludes a classical mechanism of nucleation from solution or through assembly of small nanoslabs.

Ultrasmall Zeolite Subcrystal Catalyst Devoid of Intracrystalline Diffusion Limitation for Bulk Molecule Conversion

“…Moreover, about 47.8% yield of Al-MFI-SC can be obtained (Figure a), implying the high efficiency of this synthetic strategy. The DLS measurement shows that the size of as-extracted Al-MFI-SC has a tight distribution (∼9 nm, Figure b), which is close to the smallest zeolite size range that can be obtained by the hydrothermal treatment . The Al-MFI-SC aqueous clear sample without the dissociative template is directly used, ensuring that the original morphology of the sample can be observed under a high-resolution cryogenic transmission electron microscope (cryo-HRTEM, Figures c and S1).…”

“…The DLS measurement shows that the size of as-extracted Al-MFI-SC has a tight distribution (∼9 nm, Figure 1b), which is close to the smallest zeolite size range that can be obtained by the hydrothermal treatment. 30 The Al-MFI-SC aqueous clear sample without the dissociative template is directly used, ensuring that the original morphology of the sample can be observed under a high- resolution cryogenic transmission electron microscope (cryo-HRTEM, Figures 1c and S1). In these images, Al-MFI-SC shows relatively complete short lattice stripes representing the MFI framework.…”

“…Recently, Molinero proposed, based on thermodynamic analysis and molecular simulation, that zeolite particles equal to or greater than 4 nm can remain stable in the solution during hydrothermal synthesis. 30 This certainly inspires us to keep up efforts for preparation and extraction of ultrasmall zeolite. Meanwhile, it raises the question of whether the complete elimination of intracrystalline diffusion limitations can be achieved within ultrasmall zeolite and how to illustrate this achievement.…”

“…Besides, there is a question of what is the limit of size reduction if the pursuit of tiny zeolite is the most effective strategy for enhancing the accessibility of active sites. Recently, Molinero proposed, based on thermodynamic analysis and molecular simulation, that zeolite particles equal to or greater than 4 nm can remain stable in the solution during hydrothermal synthesis . This certainly inspires us to keep up efforts for preparation and extraction of ultrasmall zeolite.…”

The Current Understanding of Mechanistic Pathways in Zeolite Crystallization

“…3 nm do not have the structure of the final zeolite but rather distinct degrees of order, which are determined by the interplay of bulk stability and interfacial forces. 365 Studies of silicalite-1 (MFI) have shown that clusters formed via the aggregation of OSDA-silica nanoparticles can transform into fully crystalline particles (Figure 19, pathway ii), which could be used to argue the existence of PNCs; 225,238,366 however, Gebauer and co-workers suggested that large clusters of silica are too stable to be considered PNCs because of their covalent nature. 290 They instead posited that these clusters should be referred to as a secondary phase.…”