Effects of Exchangeable Cations on The Thermal Behavior of Fibrous Zeolites

Intermediate phases in the natrolite series (natrolite, mesolite and scolecite) are produced by (Na, Ca)-exchange treatment, and their dehydration behaviors are investigated. The lattice parameters of these products change linearly as a function of Na₂O ratio (Na₂O/(Na₂O+CaO+K₂O) mole ratio). The dehydration behavior for each phase changes continuously with Na₂O ratio. The effect of exchangeable cations on the thermal stability of fibrous zeolites are also studied. These zeolites containing divalent cations such as Ca²⁺ and Ba², become amorphous upon dehydration, while those having only monovalent cations such as Na⁺ and K⁺ transform into dehydrated phases, and subsequently lose their crystallinity at higher temperatures. The difference in thermal stability is interpreted in terms of difference in the population and distribution of exchangeable cations in the alumino-silicate framework.