Role of alkali-metal cations and seeds in the synthesis of silica-rich heulandite-type zeolites
Clinoptilolite, the silica-rich member (Si/Al>4) of the heulandite family of zeolites, crystallizes from pure Li, Na, K, and Rb ion-containing gel systems as well as mixed Li,K, Na,K and K,Rb gels. Crystallization occurs at temperatures between 140 and 190 °C and is relatively insensitive to the nature of the silica or alumina source. Members of this family are formed over a narrow range of gel Si/Al ratio (2.5–6) and OH/SiO2 ratio (0.3–0.4 in the Na,K system and 0.6–0.9 in the Li,K system). The nature of the alkali-metal cation does not have a critical structure determining role in the synthesis but does contribute to other properties of the material including the rate of crystallization, the Si/Al ratio of the resulting crystals, the crystal size and the morphology. Potassium ions greatly increase the rate of crystallization and decrease the nucleation time. The addition of other cations to the potassium ion-containing gels slows the rate of crystallization but increases the stability of the resulting clinoptilolite crystals in the mother liquor. Sodium ions increase the Si/Al ratio of the crystals while lithium ions increase their aluminium content. Seeds (1–10 mass%) promote crystallization in the Li-, Na-, Rb- and Rb,K- containing systems, but are not necessary in gels containing K, Na,K or Li,K. In the absence of seeds, other phases co-exist or are preferred, including mordenite, phillipsite, and analcime depending on the Si/Al ratio of the gel. Crystallization time is the key parameter in preparing high purity clinoptilolite materials.