PNNL

Abstract

Zeolites are porous aluminosilicate materials that find tremendous applications in the chemical industry in separations, catalysis, ion exchange, etc. However, despite their widespread use, the reaction mechanisms occurring during zeolite growth are still unclear. Herein, we use Density Functional Theory calculations to gain insights into the thermodynamics of oligomerization, the initial steps of zeolite growth. By taking into consideration solvent and temperature effects, our results demonstrate that the growth of aluminosilicate systems is significantly more exothermic than their pure silicate counterparts. Under pH neutral conditions, water prefers to dissociate on the early-growth-stage aluminosilicate complexes rather than desorb, thus generating potential Brønsted acid sites on the oligomers. Additionally, (alumino)silicate growth pathways are evaluated in the presence of Na+ cation, as well as the Ca2+ cation for the pure silicate pathway. The presence of cations results in increasing the exothermicity of growth, with the Ca2+ exhibiting the most energetically favorable growth environment for the silicate systems. Importantly, we demonstrate through reaction extent analysis that the presence of cations modulates the speciation of the formed oligomers, with Na+ favoring linear species in addition to the generally preferred cyclic ones. Overall, this work provides a fundamental understanding of the thermodynamics of complex reaction paths that occur during early stages of zeolite growth and suggests that the initial growth steps can have significant impact on the final zeolite structure.