What is the Smallest Nano-Zeolite that Could be Synthesized?

Zeolites are crystalline microporous silicates or aluminosilicates most widely prepared by hydrothermal synthesis. Recently, few-unit cell nanozeolites have gained attention because of their potential for superior performance as catalysts, adsorbents and in membranes. A recent experimental study demonstrates that it is possible to make 4 to 8 nm diameter MFI zeolites, about ~2-to-4-unit cells, with a smart choice of structure directing agent. The smallest size zeolite that could be produced by hydrothermal synthesis is not yet known. In this work we present a simple, computationally efficient coarse-grained model that accounts for the polymerization, crystallization and dehydration processes occur in the hydrothermal synthesis of zeolites from aqueous solutions. We show that our model is able to quantitatively reproduce the experimental evolution of silica species during the polymerization from solution, and produces amorphous precursor nanoparticles with shape, composition and silica speciation in agreement with those formed in experiments. Molecular simulations and thermodynamic analysis are used to demarcate the thermal and structural stability of zeolites as a function of size. We demonstrate that MFI zeolites smaller than 2 nm would not be thermodynamically stable at temperatures of hydrothermal synthesis. Moreover, our simulations indicate that it is possible to nucleate and grow a nanozeolite as small as single unit cell (3 nm). However, other distinct more compact structures with pores smaller than the zeolites become competing structures at this small size. We conclude that 4 nm zeolites would be the smallest accessible through hydrothermal synthesis. Our study is first to establish a lower limit for the size of these promising nanomaterials.