(349y) How Do the Building Blocks of Zeolites Grow?

Despite extensive experimental and computational efforts for decades, the mechanism of zeolite formation is not fully understood due to its highly complicated nature. The microporous frameworks of zeolites are determined by their cage structures, which comprise rings containing 3-12 tetrahedral sites (Si and/or Al) bridged with oxygens. These building blocks are known to play a crucial role in zeolite synthesis but are challenging to track experimentally. Herein, we use Density Functional Theory calculations to study the formation thermodynamics of ring structures (monocycles), linked rings (multicycles), and cages, to provide insights into zeolite formation mechanisms. For pure-siliceous species, we reveal a thermodynamic preference for cages over multicycles, followed by monocycles. The difference in the formation energies increases with temperature with entropic contributions favoring the cage formation. However, the presence of aluminum atoms (at Si/Al ratio≈1) eliminates the preference among the structures. Instead, aluminosilicates show strong size-dependence, with the larger structures being favored over smaller. We also investigated the effects of cations (Na⁺, K⁺, and Ca²⁺) on the cage formation. The formation of pure silicate tetragonal prism (4-4) was thermoneutral with Na⁺, but endothermic with K⁺ or Ca²⁺. Likewise, in the presence of Na⁺ or K⁺, silicate pentagonal prism (5-5) showed exothermic formation energy, which was thermoneutral with Ca²⁺. These results demonstrate that cations can impose a size-selection effect on cages at specific sizes (4-4 and 5-5). Interestingly, cages that consist of aluminosilicates in the presence of cations were found to exhibit significantly enhanced exothermicity at sizes larger than 4-4. This work provides an in-depth understanding of the formation thermodynamics of zeolite building blocks and reveals cage size selections that can be the driving force for the final zeolite structure stabilization. Overall, our work reveals novel aluminosilicate growth behavior that can guide the experimental synthesis of zeolites with finely controlled structure.