<p>The roles of organic additives in the
assembly and crystallisation of zeolites is still not fully understood. This is
important when attempting to prepare novel frameworks to produce new zeolites.
We consider 18-crown-6 ether as an additive, which has previously been shown to
differentiate between the EMT and FAU zeolite frameworks. However, it is
unclear whether this distinction is dictated by influences on the metastable
free-energy landscape or geometric templating. Using high pressure synchrotron
X-ray diffraction, we have observed that the presence of 18C6 does not impact
the EMT framework flexibility – agreeing with our previous geometric
simulations and suggesting that 18C6 does not behave as a true geometric
template. This was further studied with computational modelling, using
first-principles comparative periodic DFT and lattice-dynamics calculations. It
is shown that the lattice energy of FAU is more stable than EMT, however this
is strongly impacted by the presence of solvent/guest molecules in the
framework. Furthermore, the EMT topology possesses a greater vibrational
entropy, being stabilised by free energy at finite temperature. Overall, these
findings demonstrate that the role of the 18C6 additive is to influence the
free-energy of crystallisation to assemble the EMT framework as opposed to FAU.
</p>
Role of preferential weak hybridization between the surface-state of a metal and the oxygen atom in the chemical adsorption mechanism
