Hydrothermal synthesis of alkali cation heulandite aluminosilicate molecular sieves

Abstract

Heulandite molecular sieves with a Si/Al ratio of 3.2 ~ 3.8 have been synthesized hydrothermally in the presence of the alkali cations Li⁺; Li⁺, K⁺; and Na⁺, K⁺ at 190 °C at autogenous pressure. They have been characterized by X-ray powder diffraction and scanning electron micrographs. Heulandites can be formed in the Li⁺ and Na⁺, K⁺ cation systems only with ~10 wt% seed crystals. But heulandite can be synthesized in the presence of Li⁺, K⁺ cations without seed crystals. The formation of heulandite-type zeolites occurs over a narrow crystallization field and depends on the Si/Al ratio, the OH⁻/Si ratio, and on the presence of seed crystals. Scanning electron micrographs show a thin-plate topology for the crystals. Heulandite is thermally unstable for calcination beyond 350 ~ 450 °C.