Title:
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An in-depth study of the crystal growth of Zeolite L
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The crystal growth of the nanoporous zeolite L has been studied in this work using
atomic force microscopy (AFM) to investigate late-phase crystallization processes
occurring on its surface. The nanometer resolution of the AFM has identified the twodimensionallayer-
by-Iayer growth mechanism of this zeolite on the {001} face, however
the {1~O} face displays a previously unreported mode of attachment for this mechanism.
The probability for growth in the long, c axis of the crystal is thousands of times more
likely than growth in the orthogonal, a direction. Growth on this face was found to occur
via the incorporation of cancrinite columns, with a measured step height of 1.2 nrn. The
frustrated a-directional growth of the crystal can occur only by the bridging connection of
two adjacent cancrinite columns, affording terraces 1.6 nm in height.
The modification of the crystal growth mechanism of zeolite L was studied when its habit
was modified with respect to the length and diameter of these hexagonal cylinder-shaped
crystals. By varying synthetic parameters such as the composition and reaction time,
controlled modifications in crystal habit could be observed. By studying the surfaces of a
vast array of crystals, the alteration in growth mechanism and defect formation were
identified. Extensive holes and cracks were observed on the surface of the {001} face
that were formed as a consequence of low supersaturation conditions. The increased
understanding of the growth mechanism of zeolite L was utilised to impart control over
the resultant crystal habit by the addition of 21-crown-7. The function of this crown ether
is likely to facilitate the lateral growth of cancrinite columns on the {1~O} face resulting in
crystals that have decreased length and increased diameter.
The first in-situ surface dissolution study of zeolite L was performed. The observation of
a high degree of friction, detected exclusively on the dissolving parts of the crystal,
enabled a detailed quantitative investigation to be carried out. This study provided
evidence about the fine structural modification of the surface during dissolution on the
{100} face. Additionally, the first methodology for calculating enthalpies for dissolution,
activation energies, orders of reaction and dissolution rates from the friction data
provided by AFM has been reported. A 1.6 nm terrace on the {10Q} face of zeolite L
under basic conditions was found to dissolve with a total enthalpy of dissolution of 216 kJ
mot", an activation energy of 25 kJ mer' and an order of reaction of 0.31. The
dissolution, rate of the terrace was found to vary when the pH, applied load and
temperature were varied.
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