Understanding initial zeolite oligomerization steps with first principles calculations

Abstract

Zeolites are porous aluminosilicate materials that find various applications in the chemical industry in separations, catalysis, ion exchange, and so forth. 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, which constitute 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 Ca 2+ cation for the pure silicate pathway. The presence of cations increases the exothermicity of growth, with Ca 2+ 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.

Seeded Stage III glass dissolution behavior of a statistically designed glass matrix

“…The impactful effect of Ca can be explained in terms of both zeolite formation kinetics and the role of Ca in gel layer formation and stability. In terms of zeolite formation, Ca has recently been shown to increase the exothermicity of growth in initial oligomerization steps in silicate systems 58 . These favorable thermodynamics may lead to faster zeolite formation kinetics.…”

“…In terms of zeolite formation, Ca has recently been shown to increase the exothermicity of growth in initial oligomerization steps in silicate systems. 58 These favorable thermodynamics may lead to faster zeolite formation kinetics. The faster formation kinetics of the zeolite are directly linked to increases in the glass dissolution rate either through an affinity effect associated with a decrease in orthosilicic acid activity [32][33][34]59 or from destabilization of a passivating gel layer.…”

On the key role of aluminium and other heteroatoms during interzeolite conversion synthesis

“…136 The formation of Si-O-Al bridges requires lower activation energies than Si-O-Si formation. 137,138 As such, dissolved Al plays a crucial role in building condensed structures. 139 Computational efforts at relevant synthesis conditions indicate that cyclization of condensation products is thermodynamically favoured and SDAs such as Na + or Ca 2+ further increase condensation tendency.…”

“… 139 Computational efforts at relevant synthesis conditions indicate that cyclization of condensation products is thermodynamically favoured and SDAs such as Na + or Ca 2+ further increase condensation tendency. 138 Of course, the kinetics of oligomerization should also be considered. Swaddle explains that cyclic oligomers cannot readily expand their coordination number beyond four (required for S N 2) 137 and so cyclic oligomers are less reactive than their acyclic peers.…”

“… 4 If not allowed by current technologies, computational studies could help to further uncover the role of dissolution thermodynamics. 138 In this regard, only indirect measurements exist. Interesting to mention is the very recent effort of Dedecek et al , who claimed to have traced the accumulation of ‘linear’, rather than cyclic, components participating in IZC assembly, from their solid state (SS) NMR study.…”