(101a) Understanding Oligomerization Steps in Zeolite Growth Using Density Functional Theory
Zeolites are aluminosilicate porous materials that are both naturally occurring and synthetically produced. Zeolites find many applications across various fields, including being used as membranes for separations, catalysts for chemicals production, etc. Being able to control the structure and chemical functionality of a given zeolite is extremely important for its application. Zeolite synthesis has been a mature field and many breakthroughs have been achieved in controlling the final structure of a zeolite. However, despite the progress that has been made in the synthesis of these materials, and their widespread use, the process of zeolite growth has not been fully understood and any advances rely heavily on trial-and-error experimentation in the lab. In this work, we use Density Functional Theory (DFT) calculations to gain a deep understanding of the energetic profiles of the initial steps of oligomerization in zeolite growth. Both thermodynamics and kinetics of silanol oligomerization steps (early zeolite growth steps) have been investigated. Since zeolite growth often occurs in the presence of a cationic species, we have evaluated these initial steps in the presence of calcium and sodium, as well as in the absence of cations. In addition, we take into account entropic contributions and the presence of a solvation environment and address temperature effects in the reaction profiles (free energy calculations). Our results demonstrate that the cations act as nucleation centers for growth bringing together silanol building blocks which dehydrate at elevated temperature to form zeolite rings. The presence of cations has a dramatic impact on the exothermicity of the reactions with calcium exhibiting an enhanced effect than sodium. Overall, this work provides a fundamental understanding of initial growth steps of zeolites.