Recently identified zeolite precursors consisting of
concentrated,
hyposolvated homogeneous alkalisilicate liquids, hydrated silicate
ionic liquids (HSIL), minimize correlation of synthesis variables
and enable one to isolate and examine the impact of complex parameters
such as water content on zeolite crystallization. HSIL are highly
concentrated, homogeneous liquids containing water as a reactant rather
than bulk solvent. This simplifies elucidation of the role of water
during zeolite synthesis. Hydrothermal treatment at 170 °C of
Al-doped potassium HSIL with chemical composition 0.5SiO2:1KOH:xH2O:0.013Al2O3 yields porous merlinoite (MER) zeolite when H2O/KOH exceeds
4 and dense, anhydrous megakalsilite when H2O/KOH is lower.
Solid phase products and precursor liquids were fully characterized
using XRD, SEM, NMR, TGA, and ICP analysis. Phase selectivity is discussed
in terms of cation hydration as the mechanism, allowing a spatial
cation arrangement enabling the formation of pores. Under water deficient
conditions, the entropic penalty of cation hydration in the solid
is large and cations need to be entirely coordinated by framework
oxygens, leading to dense, anhydrous networks. Hence, the water activity
in the synthesis medium and the affinity of a cation to either coordinate
to water or to aluminosilicate decides whether a porous, hydrated,
or a dense, anhydrous framework is formed.