Crystal structure and morphology of fully hydrated zeolite Na-A

Abstract

Zeolite Na-A with idealized composition usually stated as Na12Al12Si12O48 · 27 H2O was synthesized with various amounts of triethanolamine (TEA) between 0.25 and 2.5 mol/L, and constant amounts of sodium, silicon, aluminum, and water. The maximum crystal size increased from 9 to 28 μm with decreasing lattice parameters upon increasing amounts of TEA in the initial gel. Crystals synthesized with low amounts of TEA exhibit both {100} and {110} crystal forms while the bigger crystals made with high amounts of TEA are cubes with {100} faces only. The crystal structure of fully hydrated zeolite A, Na94Al94Si98O384 · n H2O (unit-cell composition, n refined to 243 compared with about 216 expected for the type material). was investigated by single-crystal synchrotron methods (space group Fm-3c, a = 24.550(3) Å, room temperature, R1 = 0.071) yielding a very regular framework geometry with Si—O distances between 1.599(1) Å and 1.611(1) Å, and Al—O distances between 1.719(1) Å and 1.737(1) Å. Water molecules form a sodalite-cage-like cluster in the big grc units (α cages). Other water molecules occupy a 48-fold position in the toc units (β cages). Na atoms are close to the centers of the 6-rings and in the grc units of the centers of the 8-rings.