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.
- Molecular sieves