Halil, D.; (2008) Unravelling the mechanisms of zeolite crystal growth. Doctoral thesis , University of London.

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Abstract

A combined computational and experimental study has been undertaken to understand the influence of cations on zeolite growth. Li-A(BW) and Na-J(BW) have different topologies but are synthesised from almost identical starting ingredients, the alkali metal being the only variable. A comparative study of these two zeolites therefore allows the effects of the cations upon nucleation and crystallisation processes to be directly inferred. We report the minimum energy structures of Li-A(BW) and Na-J(BW) obtained by application of an atomistic and a plane-wave DFT approach together with a synthetic study. The synthesis was prompted by inconsistencies between the calculated hydrated Na+ positions of Na-J(BW) and those determined from previous experimental work. With support of molecular dynamics simulations, we present a re-determination of the cation positions, which are found to be temperature dependent. Atomistic surface simulations suggest that Li-A(BW) crystal surfaces may be assembled by preferential attachment of single 4-rings (4MR), and Na-J(BW) surfaces by smaller monomelic and dimeric species. Simulation of aluminosilicate oligomers, by application of DFT, predict the most thermodynamically favourable oligomerisation pathways in the presence of each cation at pH and temperatures representative of different phases during zeolite growth. At higher pH we observe more pronounced cation selectivity, and a shift in the preference for high-Al to low-Al oligomers. The formation of 4MRs is reported to be thermodynamically more favourable in the presence of lithium, with smaller oligomers being more dominant in the presence of sodium, which correlates to the species required for the growth of Li-A(BW) and Na-J(BW), respectively. Entropic factors are found to drive ring formation and disfavour polymerisation, observed to have a greater contribution with increasing temperature. We propose a mechanism for 4MR formation, identified as the thermodynamically most favourable product after dimerisation. Thus, we provide evidence of how the populations of solution phase oligomers are influenced by the presence of different cations, explaining the observed "structure-directing" effect of cations.

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