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 fam ily of zeolites, crystallizes from pure Li, Na, K, and Rb ioncontainin g gel systems as well as mixed L1,K, Na,K and K,Rb gels. Crystallizati on occurs at temperatures between 140 and 190 degrees C and is relativ ely insensitive to the nature of the silica or alumina source. Members of this family are formed over a narrow range of gel Si/A1 ratio (2.5 -6) and OH/SiO2 ratio (0.3-0.4 in the Na,K system and 0.6-0.9 in the L i,K system). The nature of the alkalimetal cation does not have a crit ical structure determining role in the synthesis but does contribute t o other properties of the material including the rate of crystallizati on, the Si/Al ratio of the resulting crystals, the crystal size and th e morphology. Potassium ions greatly increase the rate of crystallizat ion and decrease the nucleation time. The addition of other cations to the potassium ion-containing gels slows the rate of crystallization b ur increases the stability of the resulting clinoptilolite crystals in the mother liquor. Sodium ions increase the Si/Al ratio of the crysta ls while lithium ions increase their aluminium content. Seeds (1-10 ma ss%) 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, includi ng mordenite, phillipsite, and analcime depending on the Si/Al ratio o f the gel. Crystallization time is the key parameter in preparing high purity clinoptilolite materials.